The fluoran derivatives are extremely useful as dye precursors for recording materials, particularly pressure-sensitive recording materials, heat-sensitive recording materials, and electro-recording materials. However, conventionally known general processes for synthesizing such fluoran derivatives have problems with respect to yield and the reaction apparatus. Therefore, it has been difficult to conduct such processes on an industrial scale.
Examples of well known general processes for synthesizing 3,6-diaminofluoran derivatives are shown by the two following schemes A and B (Beilsteil Handbuch, vol. 19, pp. 342-4). ##STR1##
However, where R and R' both represent an aryl group, the above-described two processes have serious problems. For example, in the process of scheme A, synthesis of a starting material m-hydroxytriarylamine (V) is difficult. Ordinary reaction between diarylamine (IV) and resorcin requires elevated temperatures of 200.degree. C. or higher in the presence of an acid catalyst such as phosphoric acid. Therefore, corrosion-resistant special reaction apparatuses are required. In addition, this process does not give a satisfactory yield of the end product.
An alternative process of reacting diarylamine (IV) with m-iodoanisole according to Ullmann reaction and demethylating the resulting product to obtain compound (V) has problems with respected to availability of m-iodoanisole. Futhermore, the Ullmann reaction requires high temperatures and a long reaction time. Thus, this process is not an advantageous synthesis process.
On the other hand, in the process of scheme B, the yield in the final reaction is seriously low. In addition, isolation and purification of an intended fluoran derivative is quite difficult. Thus, it is extremely difficult to industrially produce the fluoran derivatives according to either process.